Add CustomStringConvertible conformance to ExitTest.Condition and StatusAtExit. (#1073)

This PR adds `CustomStringConvertible` conformance to these data/value
types so that they are presented reasonably in expectation failure
messages or when printed.

Strictly speaking, API changes should go through a review, but there's
not really anything to review here, so with the consent of the Testing
Workgroup we can amend the exit tests proposal to retroactively include
these conformances.

See also: swiftlang/swift-evolution#2789

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

Sources/Testing/ExitTests/ExitTest.Condition.swift

@@ -27,6 +27,9 @@ extension ExitTest {
       /// The exit test must exit with a particular exit status.
       case statusAtExit(StatusAtExit)
 
+      /// The exit test must exit successfully.
+      case success
+
       /// The exit test must exit with any failure.
       case failure
     }
@@ -46,15 +49,7 @@ extension ExitTest.Condition {
   /// A condition that matches when a process terminates successfully with exit
   /// code `EXIT_SUCCESS`.
   public static var success: Self {
-    // Strictly speaking, the C standard treats 0 as a successful exit code and
-    // potentially distinct from EXIT_SUCCESS. To my knowledge, no modern
-    // operating system defines EXIT_SUCCESS to any value other than 0, so the
-    // distinction is academic.
-#if !SWT_NO_EXIT_TESTS
-    .exitCode(EXIT_SUCCESS)
-#else
-    fatalError("Unsupported")
-#endif
+    Self(_kind: .success)
   }
 
   /// A condition that matches when a process terminates abnormally with any
@@ -122,6 +117,29 @@ extension ExitTest.Condition {
   }
 }
 
+// MARK: - CustomStringConvertible
+
+@_spi(Experimental)
+#if SWT_NO_EXIT_TESTS
+@available(*, unavailable, message: "Exit tests are not available on this platform.")
+#endif
+extension ExitTest.Condition: CustomStringConvertible {
+  public var description: String {
+#if !SWT_NO_EXIT_TESTS
+    switch _kind {
+    case .failure:
+      ".failure"
+    case .success:
+      ".success"
+    case let .statusAtExit(statusAtExit):
+      String(describing: statusAtExit)
+    }
+#else
+    fatalError("Unsupported")
+#endif
+  }
+}
+
 // MARK: - Comparison
 
 #if SWT_NO_EXIT_TESTS
@@ -139,7 +157,13 @@ extension ExitTest.Condition {
   /// Two exit test conditions can be compared; if either instance is equal to
   /// ``failure``, it will compare equal to any instance except ``success``.
   func isApproximatelyEqual(to statusAtExit: StatusAtExit) -> Bool {
+    // Strictly speaking, the C standard treats 0 as a successful exit code and
+    // potentially distinct from EXIT_SUCCESS. To my knowledge, no modern
+    // operating system defines EXIT_SUCCESS to any value other than 0, so the
+    // distinction is academic.
     return switch (self._kind, statusAtExit) {
+    case let (.success, .exitCode(exitCode)):
+      exitCode == EXIT_SUCCESS
     case let (.failure, .exitCode(exitCode)):
       exitCode != EXIT_SUCCESS
     case (.failure, .signal):

Sources/Testing/ExitTests/StatusAtExit.swift

@@ -71,3 +71,19 @@ public enum StatusAtExit: Sendable {
 @available(*, unavailable, message: "Exit tests are not available on this platform.")
 #endif
 extension StatusAtExit: Equatable {}
+
+// MARK: - CustomStringConvertible
+@_spi(Experimental)
+#if SWT_NO_PROCESS_SPAWNING
+@available(*, unavailable, message: "Exit tests are not available on this platform.")
+#endif
+extension StatusAtExit: CustomStringConvertible {
+  public var description: String {
+    switch self {
+    case let .exitCode(exitCode):
+      ".exitCode(\(exitCode))"
+    case let .signal(signal):
+      ".signal(\(signal))"
+    }
+  }
+}